The effect of aging on the kidney will be investigated in rats, with emphasis on renal metabolism, particularly on renal responses to hormones and biochemical mechanism involved in the renal response. The areas of investigation are (a) the urinary concentrating mechanism, (b) calcium and phosphorus metabolism, and (c) the effect of calorie-restriction on renal metabolism. Renal function under investigation will be metabolic balance and clearance studies . The kinetic studies on calcium and phosphorus metabolism, however, will involve metabolism in the intestine and bone as well as the kidney. The hormonal action under study will be vasopressin and parathyroid hormone, and the biochemical studies of the hormonal action in the kidney will be focussed on the cyclic nucleotide system (cyclic AMP, adenyl cyclase, phosphodiesterase, protein kinase, etc.). Calorie-restriction increases the longevity of rats and also decreases the incidence of pathological (structural) changes in the kidney. Thus, the effect of calorie-restriction on renal function will be evaluated with an emphasis on animals which survive over 36 months of age (usual life-expectancy in rats on ad libitum diet is less than 24 months). The renal function to be evaluated are the glomerular filtration rate, the renal blood flow, water, calcium and phosphorus metabolism and the biochemical mechanisms of hormonal action (vasopressin and parathyroid hormone) in the kidney.